<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        /* 为元素：是我们自己用css生成的元素
        不是向h1~h6,p,img这样已经预定义号的元素
         */
    .parent::before{
        content:"前面";
        display: block;
    }
    .parent::after{
        content:"后面";
        display:block;
    }
    </style>
</head>
<body>
    <div class="parent">中间</div>
</body>
</html>